The present invention relates to a system and method for controlling a vehicle speed in which the vehicle speed is controlled by adjusting an opening angle of a throttle valve located within a throttle chamber of an intake air passage of an engine. The present invention particularly relates to a system and method for controlling the vehicle speed which are capable of controlling the vehicle speed in accordance with a magnitude of an operating variable of acceleration operator and capable of maintaining the vehicle speed at a desired cruising speed.
A Japanese Patent Application First Publication (Tokkai) Sho No. 61-1546 published on Jan. 7, 1986 exemplifies a conventional vehicle speed control system.
In the conventional vehicle speed control system, when the throttle valve control is carried out in accordance with the magnitude of operating variable of the acceleration operator, a desired cruising speed control of the vehicle can be commanded through a set switch. At this time, the operating variable by which an accelerating operator is operated is stored as a criterion of a temporary release of the cruising speed running and a target vehicle speed is set. Consequently, the opening angle of the throttle valve is adjusted so that the vehicle is run at the target vehicle speed.
However, when the cruising speed running start is commanded during the acceleration of the vehicle through the operation of the acceleration operator and thereafter the acceleration operator becomes nonoperative state, an actual opening angle of the throttle valve remarkably exceeds a throttle valve target opening angle at which the cruising speed of the vehicle is maintained constant. A long time is required for the vehicle speed to converge to the target vehicle speed. In addition, a hunting occurs in the vehicle speed during the convergence of the vehicle speed to the target vehicle speed.
This is especially remarkable in vehicle control systems in which the target opening angle of the throttle valve cannot definitely be derived on the basis of the magnitude of the operating variable of the acceleration operator but be derived from a predetermined range of the target opening angle.